You can't teleport to friends that are already in the dungeon if you die and flee. However, you can mark your spot in the dungeon before you start, so it's easier to get back in with your friends.

Just like Waterworks, if you die and flee, you have a limited time in which to get back in through the sigil, and you can still meet your friends inside. Make sure you know which realm you're in, because if you flee, it's possible you end up in a different realm and/or area (especially a crowded one).